As known, an optical code is information coded in the colour and optionally in the number and in the relative size of a plurality of elements. Linear optical codes of the bar type, for example, contain an alternating sequence of typically black bars and typically white spaces, each having a width multiple of an elementary width. Two-dimensional codes contain a grid of elements of constant shape and size, but with two or more different colours. In any case, also alphanumerical characters in general fall within the term “optical code” as used in the present description and in the annexed claims.
Generally, optical code readers comprise one or more light sources and an illumination optics for illuminating an illumination pattern on an optical code, and means for detecting the light diffused by the illuminated optical code and for generating an electrical signal whose amplitude and whose phase are representative of the colour and of the size of the optical code elements.
The electrical signal is suitably processed, for example through filters, amplifiers and digitisers, for obtaining a binary representation of the optical code, which is then decoded for obtaining the information associated with the specific optical code being read. A part of the reading of the optical code, typically the decoding, can be delegated to a host processor.
More specifically, the invention relates to optical code readers of the imager type, wherein the entire code, or the entire width of a linear code, is simultaneously illuminated, and the light diffused by the entire illuminated optical code is collected and detected through a photodetector device or sensor of linear or matrix type, respectively in the case of linear and two-dimensional optical codes.
For a good reading result, the illumination pattern in imager readers should have a well defined shape. In imager readers, moreover, the electrical signal should not be influenced by non-uniformity in the illumination of the various regions of the optical code caused for example by the emission characteristics of the illumination source(s) used, nor by non-uniformity of response among the various sensitive regions of the sensor, caused for example by the different angle at which they are illuminated by the light diffused by the optical code.
It is therefore known to associate an illumination optics with the light source(s), optics which should correct such non-uniformities.
Among the known solutions for linear optical codes, besides negative or positive lenses, lenticular arrays, holographic diffusers and prismatic arrays, the latter described for example in U.S. Pat. No. 5,504,317, may be mentioned. All these solutions provide for local repetitions of microstructures, which allow to diffuse the incident light beam only in a desired direction, at a predetermined angle. The emission lobes of two light sources can thus be overlapped, increasing the uniformity of the illumination line and improving the appearance of the scan line.
In the above described known solutions, an illumination pattern of uniform intensity is provided to the expenses of a reduced overall efficiency of the reader since an area much larger than the area actually framed by the reader receiving optics is typically illuminated, as it can be seen in FIG. 15. Such a figure diagrammatically shows the irradiance profile generated on the optimum focus plane and at close reading range by two LED (Light Emitting Diode) light sources 100a, 100b having associated respective conventional optics 101a, 101b, and the relation thereof with the field of view 102 of the receiving optics. From the figure, moreover, it may be noted that at close reading range, the reader efficiency is even lower and the reading line is poorly uniform. At all reading distances, moreover, the side edges of the illumination line are poorly defined, so the reader operator does not have a clear indication of the angular extension of the reading area.
GB 2 225 659 A teaches to associate each of a pair of LEDs (Light Emitting Diodes) with a lens or lens portion for focusing the light in a direction parallel to an optical code arranged in a predetermined position with respect to the reader, and a lens or lens portion for focusing the light in a direction orthogonal to the optical code, in order to obtain a thin strip-like illumination line, wherein the illumination at the ends of the strip can be stronger than at the centre. Each lens or lens portion for focusing in the direction parallel to the code exhibits two regions with different radius of curvature. In another embodiment, such a document provides for a single LED and a lens or lens portion which is concave in a central portion and convex in the two side regions, with a radius of curvature that gradually decreases towards the lens ends. Also in this case, an area much larger than the area actually framed by the reader receiving optics is illuminated.